1. Field of the Invention
The present invention relates to compression systems for supplying compressed fluid and more specifically relates to a compression system which provides for efficient load sharing between at least two compressors without requiring an external controller for measuring the load-sharing operation.
2. Brief Description of the Prior Art
Compressed fluids such as air are typically used as the power source for equipment and tools in facilities such as manufacturing plants and construction sites. In such facilities, the demand for compressed fluid may fluctuate dramatically. In order to satisfy this continuously changing demand for compressed fluid, compression systems may be arranged in a sequencing or load-sharing configuration whereby two or more compressors are connected in parallel and have a total capacity which is large enough to meet the maximum load demands of the facility. These sequencing systems generally modify the volume of compressed fluid produced by the system by increasing or decreasing the number of compressors running at any one time. When the demand for compressed air increases, additional compressors are started up and placed "on-line." In contrast, when the demand for compressed fluid at the facility decreases, such as during a shift change, it is generally desirable to shut down one or more of the compressors because the demands on the system can be satisfied by fewer compressors. The use of sequencing systems generally results in a tremendous costs savings because the system only produces as much compressed fluid as is required by the facility. These systems also alternate the load-sharing duties of the individual compressors, thereby extending the life of the individual compressors and reducing maintenance costs.
One method for sequencing operation of a plurality of compressors is disclosed in U.S. Pat. No. 4,580,947. In one preferred embodiment, the '947 patent discloses a method of controlling the operation of a plurality of compressors, wherein a control system has a compressor control loop and a capacity control loop. The capacity control loop is further divided into a first loop for controlling a compressor which is to be stopped first and a second sub-loop for controlling the other compressors. The unloading is conducted first by the first sub-loop for controlling the compressor due to be stopped while the on-loading is conducted first by the second sub-loop for the other compressors.
Commonly assigned U.S. Pat. No. 5,343,384 discloses methods and apparatus for controlling a system of compressors to achieve load sharing. In accordance with one particular preferred embodiment, load sharing in a compression system is achieved by providing a program stored in a computer which sends signals to, and receives data from, microcontrollers located at each respective compressor. The respective microcontrollers communicate with the computer by way of a bidirectional network so that certain functions of the respective microcontrollers may be controlled by the program in the computer. One compressor, designated as the lead compressor, furnishes its operating parameters, via the computer, to all the other, lag compressors. The operating parameters include inlet valve position and bypass valve position of each compressor whereby the microcontroller for the compressor controls the actuation of both valves. When the system demand decreases, one or more compressors are gradually unloaded, stopped and then moved to an "off-line" status. When the system demands increase, one or more compressors are first started, and then gradually loaded before going on-line. Compressors go both on-line and off-line subject to certain time delays so that compressors are gradually added to, or removed from, the load. To equalize running time, all compressors in the system may undergo a periodic rotation and compressors go off-line in reverse order that they came on-line.
The methods and apparatus disclosed in the '384 patent have generally improved the performance of fluid compression systems by providing numerous benefits such as energy savings, extending the life of the individual compressors and reducing maintenance costs. However, certain preferred embodiments disclosed in the '384 patent utilize an external controller or computer. The addition of the external computer or controller to manage the load-sharing of a compression system adds to the overall complexity and cost of the system. Yet, there are certain instances where the compression system comprises only two compressors so that the addition of an external controller or external pressure sensor is neither necessary nor cost effective.
Thus, there is a need for a compression system capable of sequencing the operation of two compressors without using additional external controllers and sensors, whereby when two compressors are connected to the same air system and their pressure bands are substantially equal, setting one compressor to "lead" and the other to "lag" with a "lag offset" will result in a simple sequence operation.